在最近做开发的时候，刚好用到了这两个方法，细研究了一下两者的区别。
我们一般长用的是postDelayed，其实很简单，就是用Handler把message放到消息列队里，让主线程去执行。
主要是postOnAnimationDelayed，第一次用，在做嵌套滑动的时候，需要去处理fling，用到了这个函数。
public void postOnAnimationDelayed(Runnable action, long delayMillis) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
attachInfo.mViewRootImpl.mChoreographer.postCallbackDelayed(
Choreographer.CALLBACK_ANIMATION, action, null, delayMillis);
} else {
// Postpone the runnable until we know
// on which thread it needs to run.
getRunQueue().postDelayed(action, delayMillis);
}
}
调用了ViewRootImpl中Choreographer的postCallbackDelayed函数
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
if (DEBUG_FRAMES) {
Log.d(TAG, "PostCallback: type=" + callbackType
+ ", action=" + action + ", token=" + token
+ ", delayMillis=" + delayMillis);
}
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
final long dueTime = now + delayMillis;
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
scheduleFrameLocked(now);
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}
}
private void scheduleFrameLocked(long now) {
if (!mFrameScheduled) {
mFrameScheduled = true;
if (USE_VSYNC) {
if (DEBUG_FRAMES) {
Log.d(TAG, "Scheduling next frame on vsync.");
}
// If running on the Looper thread, then schedule the vsync immediately,
// otherwise post a message to schedule the vsync from the UI thread
// as soon as possible.
if (isRunningOnLooperThreadLocked()) {
scheduleVsyncLocked();
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
msg.setAsynchronous(true);
mHandler.sendMessageAtFrontOfQueue(msg);
}
} else {
final long nextFrameTime = Math.max(
mLastFrameTimeNanos / TimeUtils.NANOS_PER_MS + sFrameDelay, now);
if (DEBUG_FRAMES) {
Log.d(TAG, "Scheduling next frame in " + (nextFrameTime - now) + " ms.");
}
Message msg = mHandler.obtainMessage(MSG_DO_FRAME);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, nextFrameTime);
}
}
}
我们看主要逻辑，其实就是把Message设置成异步消息，异步消息简单来说就是这类消息会优先执行，不会受屏障的影响。具体可以看Android消息循环机制（二）Meassage、MessageQueue源码解析